1. Field of the Invention
This invention relates to an identification element comprising a layer of an anisotropic magnetic material exhibiting a preferred axis, high permeability and low coercive force (soft magnetic material), which on the application of an external alternating magnetic field generates a characteristic signal which is at its maximum level when the preferred axis of the identification element is oriented parallel to the external alternating magnetic field. The identification element further comprises a layer of a magnetic material of low permeability and high coercive force (semihard or hard magnetic material) which is arranged on the soft magnetic material or in the immediate proximity thereof, with the semihard or hard magnetic material having regions differing in magnetization. The present invention further relates to a method of manufacturing such an identification element.
2. Background Art
Electronic article surveillance as a means for preventing inventory losses in department stores and warehouses is becoming increasingly popular. A plurality of different security elements are known in the art. A particularly advantageous embodiment of a security element is known from European patent specification EP 0 295 028 B1. This patent specification describes labels referred to as thin-film labels. These labels comprise a thin layer of a soft magnetic material lying preferably in the .mu.m range. The layer is applied to a substrate using, for example, a physical deposition method under vacuum conditions.
Thin-film labels exhibit an anisotropic structure. Anisotropic means that the soft magnetic layer of which the thin-film labels are formed has a preferred axis. In practice, the anisotropic structure will be noticed in that the characteristic signal remitted by the thin-film label in response to an interrogating field is at a maximum level when the interrogating field and the preferred axis are in parallel alignment; by contrast, the signal will disappear when the preferred axis and the interrogating field are normal to each other.
An analogous behavior is also present in security elements referred to as strip security elements which comprise a soft magnetic material in strip form. Here, too, the characteristic signal is at a maximum level when the interrogating field and the strip are in parallel alignment, the signal disappearing in the presence of an orientation perpendicular to each other. It will be understood that the strip security element may also be a length of drawn wire.
A plurality of different methods for detecting the presence of security elements in an interrogation zone are known in the art. Thus it is suggested in European patent specification EP 123 586 B to emit into the interrogation zone, in addition to two interrogating fields with the frequencies f1 and f2 in the kilohertz range, a field with a frequency that lies in the hertz range. The two interrogating fields with the frequencies f1 and f2 incite a security element present in the interrogation zone to emit a characteristic signal with the intermodulation frequencies n.multidot.f1.+-.m.multidot.f2 (where n, m=0, 1, 2, . . . ). The low-frequency interrogating field causes the security element to be driven from saturation in one direction into saturation in the other direction at the clock rate of this particular field. Hence the characteristic signal occurs cyclically at the frequency of the low-frequency field.
As an alternative solution, it is also known to use only one interrogating field in the kilohertz range for excitation of the security element, with the characteristic signal of the security element occurring again at the clock rate of a low-frequency field cycling the soft magnetic material between the two states of saturation.
From published international application WO 97/04338 it is known to configure a security element in such fashion that apart from serving the function of protecting an article against pilferage, it has the added capability of identifying the suitably protected article. For this purpose, the semihard or hard magnetic material is magnetized with a nonuniform field pattern. By reason of the field pattern of the permanently magnetizable material, the characteristic signal of the soft magnetic material can be varied selectively in a wide variety of ways.